1. Field of the Invention
The present invention relates to a semiconductor apparatus using a crystalline metal oxide film as a dielectric film and a method of manufacturing the semiconductor apparatus.
2. Description of Related Art
Silicon nitride (Si3N4) films or silicon oxide (SiO2) films have hitherto been used as dielectric films which are used to make a capacitor device and the gate insulating films of transistors in a large-scale integrated circuit (hereinafter called “LSI”).
As the LSI becomes increasingly denser, the area occupied by its capacitor device needs to be reduced. On the other hand, aiming to diversify the application and reduce the cost of the capacitor device so as to be suited for more sophisticated functions of the LSI, a capacitor so far external to the LSI has come to be incorporated into the LSI, exhibiting a tendency that the capacitance in the LSI is increasing. In order to ensure the capacitance requirements while limiting the occupancy of the capacitor device, it is effective to use a material having a high dielectric constant as a dielectric film for forming the capacitor device.
As dielectric films having a high dielectric constant, attention has been paid recently to tantalum oxide (Ta2O5), hafnium oxide (HfO2), zirconium oxide (ZrO2), aluminum oxide (Al2O3), and the like. Development has been made positively on their applications not only to gate insulating films but also to dielectric films as capacitor devices. These materials are used generally in an amorphous phase. For example, although hafnium oxide (HfO2) and the like are easy to crystallize, they are often used with an element such as silicon (Si) added thereto in order to prevent their crystallization.
The typical dielectric constants of these materials are shown below. These are approximate values obtained in the amorphous phase, since the dielectric constant actually depends on the impurity concentration in a film and the film density, attributable to a material of which the film is formed.
TABLE 1Name of materialdielectric constantsilicon oxide (SiO2)approx. 4silicon nitride (Si3N4)approx. 7aluminum oxide (Al2O3)approx. 9zirconium oxide (ZrO2)15 to 22hafnium oxide (HfO2)15 to 22tantalum oxide (Ta2O3)22 to 30
As a material exhibiting even higher dielectric properties than the above-mentioned materials, a crystalline dielectric film is known. For example, strontium titanate (SrTiO3, hereinafter abbreviated as STO), barium titanate (BaTiO3, hereinafter abbreviated as BTO), barium strontium titanate (BaxSr1-xTiO3, hereinafter abbreviated as BST) being a crystal mixed with BTO, zirconium lead titanate (PbZrxTi1-xO3, hereinafter abbreviated as PZT) having superior ferroelectric properties, and the like have been under research and development, and on the basis of that, studies on their physical properties have been in progress toward their practical application.
These crystalline dielectric films have a crystal structure called a perovskite structure, and their dielectric constant is known to be dependent on their crystallinity (see, e.g., the following Non-Patent Document 1).
[Non-Patent Document 1] Tsuyoshi HORIKAWA, Noboru MIKAMI, Hiromi ITO, Yoshikazu OHNO, Tetsuro MAKITA, Kazunao SATO, “(Ba0.75Sr0.25)TiO3 Films for 256 Mbit DRAM”, IEICE (The Institute of Electronics, Information and Communication Engineers) TRANS ELECTRON, Vol. E77-C, No. 3, pp. 385-391, 1994
In the perovskite structure, better crystallinity exhibits a higher dielectric constant. This is understood as the phenomenon of ion polarization. Since the dielectric constants of the crystalline dielectric films respectively exhibit a large dependence on crystallinity, it is hard to give their dielectric constants summarily. However, if the films are so crystalline that they exhibit such superiority as to be highly dielectric thin films, their dielectric constants are approximately 50 to 100.
When a capacitor device having a high capacitance density is to be formed by using such a crystalline dielectric film, a film having satisfactory crystallinity must be formed. To do so, two requirements must be met.
The first requirement is to form the film at a sufficiently high temperature, which is a requirement common to any crystalline growth. Although the relation of temperature with crystallinity depends on a material used, there have been many reports that in the case of STO and BST films, their film forming temperature is generally about 500-800° C., including the subsequent heat treatment.
The second requirement is the lattice matching of the film with a surface material of a film forming substrate. Considering the formation of a capacitor device using a crystalline dielectric film from this viewpoint, a material exhibiting satisfactory lattice matching with the crystalline dielectric film is optimal as the surface material of a lower electrode. Generally, a metal material, such as platinum (Pt) or ruthenium (Ru), or a conductive oxide film, such as a ruthenium oxide film (RuO2), a strontium ruthenium oxide film (SrRuO3), or the like is used. Examples have been disclosed, which indicate the importance of lattice constant and lattice matching with an underlayer (see, e.g., Patent Document 1).
[Patent Document 1] Japanese Patent Application Publication No. 11-204745
However, even if such a material exhibiting satisfactory lattice matching is used to form the lower electrode, the crystalline dielectric film is generally known to have a tendency that its crystallinity is unsatisfactory initially, but it gradually improves thereafter in the film formation through its self lattice matching. Therefore, the film exhibits a low dielectric constant in its initially grown, less crystalline region and a high dielectric constant in its upper layer with improved crystallinity.
A method of forming a capacitor device aiming to decrease the surface area of an LSI involves forming the capacitor device between layers that have been formed after an interconnect process. By doing so, other devices such as, e.g., transistors can be formed even below the capacitor device, and hence such a method is effective. Also, the formation of a capacitor device between layers allows easy formation of a MIM (Metal Insulator Metal) structure in part of which upper and lower electrodes or existing interconnections are used. This implements a sufficiently low interconnection resistance, and hence is advantageous in high-frequency device applications.
However, in forming the capacitor device after the interconnect process, the maximum device forming temperature is restricted, and hence the device must be formed below 350-400° C., in consideration of the problems of the reliability of the interconnections and the performance fluctuations of the other devices. However, at a temperature within the above-mentioned range, the crystallinity of the crystalline dielectric film is impaired as compared with that formed at high temperature, and hence the dielectric constant is decreased.
The crystalline dielectric film having a perovskite structure exhibits a superior dielectric constant, as mentioned above. In order to obtain such a crystalline dielectric film, technologies have been developed Which involve methods using organic materials, such as a chemical vapor deposition (CVD) method (see, e.g., Patent Document 2), an atomic layer deposition (ALD) method, and a sol-gel process (see, e.g., Patent Document 3).
[Patent Document 2] Japanese Patent Application Publication No. 2002-353208
[Patent Document 3] Japanese Patent Publication No. 3152135
While these film forming methods employing organic materials have a feature of superior step coverage, they also have a problem that organic components, such as carbon and hydrogen, contained in a film forming material remain in the film. For example, in forming a dielectric film using an organic material, it is generally known that impurities such as carbon and hydrogen increase leakage current. Additionally, the presence of such impurities impedes crystal growth and invites the dielectric constant to decrease.
Since the organic components remain more noticeably in low-temperature film formation, it is a must to form the film at high temperature. However, it becomes difficult to form the capacitor device between the layers produced after the existing interconnect process, since high-temperature processing is involved. Meanwhile, film formation based on a sputtering method (see, e.g., Patent Document 4) is effective in crystal growth in the sense that it can avoid the influence of the organic components that remain.
[Patent Document 4] Japanese Patent Application Publication No. 2003-224123
The crystallinity of the crystalline dielectric film is unsatisfactory initially, but it gradually improves thereafter in the film formation through self lattice matching. The film exhibits a low dielectric constant in its initially grown, less crystalline region, whereas it exhibits a high dielectric constant in its upper layer with improved crystallinity. Furthermore, the crystalline dielectric film is known to have a tendency that its current leakage characteristics depend on the crystallinity. For example, when attention is paid to BST as a crystalline dielectric material, many examples of experiments have been reported in which the dielectric constant increases with increasing film forming temperature, and so does leakage current (see, e.g., Non-Patent Document 2). In other words, the initially grown film exhibits a quality that decreases the leakage current. This tendency becomes more noticeable at lower crystalline dielectric film forming temperatures. A similar tendency is observed also in an RF sputtering method, which is known as a superior low-temperature film forming method.
[Non-Patent Document 2] Tsuyoshi HORIKAWA, Junji TANIMURA, Takaaki KAWAHARA, Mikio YAMAMURA, Masayoshi TARUTANI, Kouichi ONO, “Effects of Post-Annealing on Dielectric Properties of (Ba, Sr)TiO3 Thin Films Prepared by Liquid Source Chemical Vapor Deposition”, IEICE (The Institute of Electronics, Information and Communication Engineers) TRANS ELECTRON, Vol. E81-C, No. 4, pp. 497-504, 1998
When the capacitor device is to be formed using the crystalline dielectric film as a capacitor insulating film, the crystallinity of the crystalline dielectric film (e.g., a BST film or the like) differs at a lower electrode interface and at an upper electrode interface, with the crystallinity being more satisfactory at the upper electrode interface than at the lower electrode interface. From this, the dependence of the current leakage characteristics on the direction in which bias is applied on the electrodes. A direction in which electrons are injected from the lower electrode, i.e., a state in which a positive bias is applied on the upper electrode is denoted as a “positive bias”, whereas a state in which a negative bias is applied on the upper electrode is denoted as a “negative bias”.
The structure of a capacitor device whose current leakage characteristics were evaluated is shown in FIG. 1, which is a cross section showing a general configuration. The directions of electron injection based on the biasing directions are shown in FIG. 3. An example of the dependence of the current leakage characteristics on biasing is shown in FIG. 4.
As shown in FIG. 1, the capacitor device has a structure in which a BST film 130 is interposed between a lower electrode 120 and an upper electrode 140. As the BST film 130, a film RF-sputtered at 400° C. or lower was used, and platinum was used to make the lower electrode 120 and the upper electrode 140. Also, as shown in FIG. 3, as to the relationship between the biasing direction and the direction of electron injection, electrons are injected from the lower electrode 120 to the upper electrode 140 at positive bias, whereas electrons are injected from the upper electrode 140 to the lower electrode 120 at a negative bias. The current leakage characteristics of such a capacitor device were evaluated, and the result is shown in FIG. 4.
As shown in FIG. 4, the current leakage characteristics exhibit a clear dependence on the biasing direction. At a positive bias, a mild upward curve is seen in the leakage current/capacitance density as the voltage increases, whereas at a negative bias, it is seen that the leakage current/capacitance density suddenly increases as the voltage (absolute value) increases. This demonstrates a tendency due to the crystallinity of the BST film.
Additionally, a drawing of a cross sectional, transmission electron microscope (TEM) photograph of the BST film 130 on which the above-mentioned measurements were made is shown in FIG. 2. The BST film 130 in its initial phase of formation, i.e., at an interface with the lower electrode 120 is amorphous, whereas its upper layer, i.e., at an interface with the upper electrode 140, is polycrystalline.
In the crystalline dielectric film, such as a BST film, formed at low temperature, the dependence of its current leakage characteristics on biasing is noticeably seen from a difference in crystallinity between the region grown initially and the upper layer grown thereafter in the film formation. Thus, deterioration of the current leakage characteristics at a negative bias imposes the problem of reliability. Also, its dependence on the biasing direction imposes a problem such as signal distortion at an RF (radio frequency) band. It is for this reason that improvements of the current leakage characteristics have been called for. And, if the current leakage characteristics can be improved by a method not inviting a decrease in the unit capacitance of the capacitor device, such a method is suitable since the performance of the crystalline dielectric film is not impaired.